This invention relates generally to an electrophotographic printing machine, and more particularly concerns a corona generating apparatus for charging a photoconductive surface to a substantially uniform potential.
In a typical electrophotographic printing machine, a photoconductive surface is electrostatically charged substantially uniformly thereover and then exposed to a light pattern of the image being reproduced. The light pattern discharges the electrostatic charge in the areas where the light strikes the photoconductive surface. As a consequence the remaining charge on the photoconductive surface forms an electrostatic charge pattern in image configuration, i.e., an electrostatic latent image. The electrostatic latent image may then be developed by contacting it with finely divided electrostatically attractable material, such as toner particles. The toner particles adhere electrostatically to the photoconductive surface in a pattern corresponding to the latent image recorded thereon. Thereafter, the developed image is transferred to a suitable sheet of support material, such as paper, amongst others, which is secured releasably to a transfer member. The powder image transferred to a sheet of support material is subsequently suitably affixed thereto to form a permanent print thereof.
With the advent of multi-color electrophotographic printing involving the utilization of various components adapted to produce a series of electrostatic latent images in which each image represents a particular color in the original, there is a need to transfer successive single color toner powder images onto the sheet of support material. Incidentally, it will be appreciated that a black and white or single color reproduction, such as a red, cyan, magenta or yellow print, requires only a single toner powder image, whereas a multi-color reproduction may necessitate a plurality of differently colored toner powder images. It, therefore, follows that the amount of toner particles required in the formation of a multi-color copy may be appreciably greater than that required for the production of a single color copy. For example, in a three color subtractive system, cyan, magenta, and yellow toner particles are successively transferred to the support material. Thus, the formation of a multi-color copy from a colored original requires more toner particles than that utilized in the formation of black and white copy. It is evident, therefore, that this substantially increases the potential amount of contamination from dust and toner particles.
In electrophotographic printing, it is necessary to deposit a substantially uniform potential on the photoreceptor surface to insure that electrostatic charges may be selectively dissipated in accordance with the light pattern of the image being reproduced. This may be achieved by such prior art devices as the corona generating device described in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. A corona generating device generally includes a corona discharge electrode, such as a coronode wire, surrounded by a conductive shield. The corona discharge electrode is adapted to be supplied with a DC voltage of sufficient magnitude to create a corona current flow from the electrode to the surface of a photoreceptor spaced therefrom. Another type of corona generating apparatus particularly suitable for use in multi-color electrophotographic printing is a corona generating device such as is described in U.S. Pat. No. 2,778,946 issued to Mayo in 1957. A typical corona generating device includes a corona discharge electrode such as coronode wire, surrounded by a conductive shield with a grid interposed between the coronode wire and the photoconductive surface. In a corona generating device, the shield is normally at ground potential and the grid is operated at some predetermined potential between the discharge electrode voltage and ground voltage. The grid controls the charge applied to the photoconductive surface.
Although the corona generating device or scorotron is advantageously utilized to substantially uniformly charge a photoconductive surface, it is suitably adapted for various other applications. For example, corona generating device electrostatically transfer a powder image from a photoconductive surface to a sheet of support material as well as remove background toner particles therefrom, and preclean corona generating device neutralize the charge on toner particles adhering to the photoconductive surface after the transfer of the powder image to the support material. However, a disadvantage of corona generating devices is their sensitivity to the accumulation of dust and toner particles. Dust or toner particles adhering to the coronode wires will decrease the corona current generated therefrom as the density of particle accumulation increases. Contrawise, dust or toner particles adhering to the grid wires will increase the corona current generated therefrom as the density of particle accumulation increases. However, the increase in current produced by particle accumulation on the grid wires is not inversely proportional to the decrease in current produced by particle accumulation in the coronode wires. Thus, the performance of a corona generating device remains sensitive to dust and toner particles.
Heretofore, various prior art devices have been developed for cleaning corona generating devices. By way of example, IBM Technical Disclosure Bulletin, Volume 11, Number 8 of January 1969 describes cleaning pads surrounding the coronode wires of a corona generating device. The cleaning pads are reciprocated along the length of the coronode wires to remove dust and toner particles accumulated thereon. Similarly, copending application Ser. No. 245,306, filed in 1972, now abandoned describes cleaning pads surrounding the coronode wires and contacting the interior surface of the shield surrounding the coronode wires. Once again, the cleaning pads are reciprocated along the length of the coronode wires and shield to remove dust and toner particles, accumulated thereon. However, neither of the foregoing approaches describe cleaning of the grid wires used in a corona generating device.
Accordingly, it is a primary object of the present invention to improve the cleaning of a corona generating apparatus.